The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Plasma arc torches, also known as electric arc torches, are commonly used for cutting, marking, gouging, and welding metal workpieces by directing a high energy plasma stream consisting of ionized gas particles toward the workpiece. In a typical plasma arc torch, the gas to be ionized is supplied to a distal end of the torch and flows past an electrode before exiting through an orifice in the tip, or nozzle, of the plasma arc torch. The electrode has a relatively negative potential and operates as a cathode. Conversely, the torch tip constitutes a relatively positive potential and operates as an anode during piloting. Further, the electrode is in a spaced relationship with the tip, thereby creating a gap, at the distal end of the torch. In operation, a pilot arc is created in the gap between the electrode and the tip, often referred to as the plasma arc chamber, wherein the pilot arc heats and ionizes the gas. The ionized gas is blown out of the torch and appears as a plasma stream that extends distally off the tip. As the distal end of the torch is moved to a position close to the workpiece, the arc jumps or transfers from the torch tip to the workpiece with the aid of a switching circuit activated by the power supply. Accordingly, the workpiece serves as the anode, and the plasma arc torch is operated in a “transferred arc” mode.
One of two methods is typically used for starting a plasma arc torch for initiating the pilot arc between the electrode and the tip. In a first method, commonly referred to as a “contact start,” the electrode and the tip are brought into contact and are gradually separated, thereby drawing an arc between the electrode and the tip. The contact start method allows an arc to be initiated at much lower potentials since the distance between the electrode and the tip is much smaller.
In the second method, commonly referred to as a “high frequency” or “high voltage” start, a high potential is applied across the electrode and the tip, which do not make physical contact with each other, to generate a plasma arc. The process begins by supplying a pre-flow gas to the plasma chamber. Electric current (called pilot current) is then applied across the electrode and the tip to sustain the plasma arc in the gap between the electrode and the tip. The pre-flow gas forces the pilot arc out of the tip orifice, thereby facilitating arc transfer to the workpiece. When current is sensed on the workpiece, the tip is removed from the electric circuit. Thereafter, an operating current is supplied between the electrode and the workpiece to sustain the plasma arc between the workpiece and the electrode. The pre-flow gas is then switched to a plasma gas, which is ionized to generate the plasma stream for cutting, welding or gouging etc. A shield gas is also typically supplied to stabilize the plasma stream.
Application of high frequency and high voltage across the electrode and the tip, however, causes electromagnetic interference (EMI) in the surrounding environment. Moreover, the tip is subject to repetitive pilot current during arc transfer and is thus susceptible to wear. Further, the arc transfer by the conventional method is not reliable.